Please Help - Engine Miss at Idle but only with AC/Defroster on

I am a college student and need your help very badly.

I have a 2002 Taurus SEL 3.0L Duratec engine, (86,000 miles). The car has a very bad and annoying engine miss at idle, most noticeable when in drive, stopped at a traffic light. The miss can be felt through the entire car.

The Service Engine light went on with the error codes PO303 (Cylinder 3 misfire) and PO316 (Misfire in first 1000 revolutions).

I had the spark plugs replaced along with the spark plug wires. These changes did not resolve the problem. I was then told to replace the coil pack. That seemed to fix the problem for a short period of time but the problem came back worst then ever. (It may have been that I did not have the air conditioner on after the coil pack was replaced – see comment below)

I then noticed an interesting fact. The engine miss would occur whenever I was stopped and idling and also when I had the AC/Defroster on. When AC/Defroster was turned off, the miss immediately went away. I have tested this many times and it is very consistent. The car seems to run fine as long as I don't turn the air conditioner/Defroster on, otherwise it starts to shake and miss very badly, especially at an idle. The engine does not miss if I just turn the heater or vent on, just the AC/Defroster.

Seems very strange that turning the AC system on, could cause the engine to misfire.

Anybody have any suggestions. I don't have the money to continue to replace parts. Please help.

Thanks in advance.

Try cleaning IAC,MAF,and oxygen sensors with CRC MAF cleaner.
Replace Air filter if bad.

When theres a demand for pwr it's going to need more Air + fuel.

The fuel delivery seems tobe working with Air Cond off and giving it gas.

The problem is during Idle + The IAC controls the Air during Idle mode.

Is the Chk Eng Light still on after replacing wires + plugs?

MCGIVER

Does the AC give you problems when at higher than 2000 rpm or 55+ mph or accelerating? Does the ac cool properly? Does it sound like it's working too hard, not turning right, maybe even starting to smell burnt? That would indicate a seizing ac clutch, but not likely your problem. I'm wondering out loud about the ac switches that feedback to the pcm somehow causing your problem, but I can't quite make the clear connection. If your ac isn't working, I'm wondering if your ac clutch field coil would be the problem by not telling the PCM it's on. I'm not sure. Hopefully, Rod will respond.

I'd first suggest making sure the plug wires are all snapped on tightly at both the plugs and the coil towers. Make sure the electrical connector is on tight to the coil pack assembly as well. I've seen that come loose and cause a pretty significant misfire as well.

Do you still have a check engine light? If so, what does the code indicate? If it's showing a misfire for a particular cylinder, pull the spark plug from that cylinder and see if it appears wet from fuel or abnormal in color.

If you are confident that the wires are good, plugs are good, and the electrical connection to the coil pack is good, then try putting an inline spark tester inline with each wire (one at a time) and look for a cylinder that isn't firing. If all 6 cylinders appear to be firing with the A/C on, but the miss is still present, then verify that all vacuum hoses and air intake tubing is properly installed and not cracked or split.

If still nothing shows up, then you may want to consider swapping around fuel injectors if you have a misfire code for a particular cylinder to see if the misfire follows the injector.

-Rod